Centrifugal separators are well known for separating solids from liquids in a slurry feedstream. Particularly, such separators are known for high-volume separation of drill cuttings from drilling fluid in returned drilling muds. The ability to remove said solids is instrumental in the reuse and recycling of at least the liquid portion of the drilling mud.
In a typical drilling operation, drilling mud is pumped down the bore of a drill string to a drill bit. The mud acts to lubricate and carry drill cuttings to surface as the drilling mud returns through an annulus between the wellbore and the drill string. The drilling mud is first passed across a shale shaker, as is known in the art, to separate large solids therefrom. Smaller particulates pass through the screen of the shale shaker and remain in the fluid. Conventional centrifugal separators are used thereafter to remove particulates from the fluid, where the solids content of the fluid is typically less than the fluid content.
The larger solids which are rejected by the shale shaker typically retain some liquid portion of the drilling fluid. In many cases, the rejected solids, from the shale shakers, are mixed with an absorbent material such as sawdust to absorb remaining liquid therein which is sent for disposal, resulting in a loss of the liquid. The loss of even small amounts of liquid results in increased operational costs as well as an increased environment hazard. This may be of particular importance with respect to oil-based drilling muds or water-based drilling muds where the ability to strip liquid from a high-solids reject would result in significant cost savings.
In other cases, the rejected solids may be diluted with additional liquid, such as diesel, to decrease the solids content to permit separation of the solids therefrom using conventional vertical or horizontal centrifugal separators.
Vertical basket filter centrifuges are known for separation of solids from liquids, however such apparatus typically permit fine solids, such as solids less than 200 um, to pass through the basket filter with the recovered liquid. Contamination of the drilling fluid by the fine solids results in a drilling fluid which is not clean enough for reuse without further treatment.
Known horizontal centrifugal separators generally have a large footprint and require pumps to feed liquid, and/or require augers to feed solids, into the centrifuge. In many cases, such horizontal separators are only suitable for slurries having a relatively high liquid content. Slurries having relatively large solid contents, such as those containing about 90% solid, may be unpumpable.
One such large, horizontal centrifuge is that taught in Canadian application 2,626,814 to Wick. Rejected solids from a shale shaker are centrifuged in the horizontal centrifuge to separate liquid remaining therein from the solids. The rejected solids have a relatively high solids content of up to 70% or more. The rejected solids are fed into a drum using a feed auger which replaces a feed tube through which fluid would normally be pumped in a conventional horizontal separator. The centrifuge comprises the drum, driven for rotation about a horizontal axis, and an auger which is disposed within the drum. The auger within the drum is rotatable about the horizontal axis and can be rotated at a different speed than the drum. The drum has a tapered portion, the radius becoming smaller at one end than at an opposing end for forming a low bowl angle. The auger has a small clearance between the flights of the auger and the housing. The flighting assists in moving solids from an inlet end to an outlet end while liquid, under the influence of a radially outward force, flows in the small clearance in a direction opposite to the solids. Thus, the liquid is separated from the solids.
There is interest in a centrifugal separator which has a small footprint to permit use in a wide variety of applications and which does not require pumps or augers to transport the feed through the separator. Further, there is interest in a centrifugal separator which recovers liquids from returned drilling mud, the liquid having a rheology suitable for reuse in a drilling operation. More particularly, there is interest in a centrifugal separator which is also capable of handling rejected solids, such as from a shale shaker, the rejected solids having a relatively high solids concentration, typically of up to 70% or greater.